Sondrestrom observations show that a characteristic F-region signature of the interaction between the magnetosphere and ionosphere is a narrow band of elevated electron temperatures. Its location is associated with the ion convection reversal in the morning and evening convection cells. Typically, near 500 km altitude, the temperature is 3500 to 4000 K. However, on 24 April 1983, a geomagnetically very active day with Kp values of 6-during the period of interest, the electron temperature reached 6000 K in the afternoon convection reversal. The ion velocities were between 1 and 2 km/s on both sides of the reversal. There was considerable soft particle precipitation and a large downward heat flux of 0.3 erg/cm2-s at 450 km. These high temperature electrons then transferred 0.5 erg/cm2-s to ions and neutrals between 175 and 550 km, which represents a very sizeable perturbation to the thermosphere.